Contact adhesives based on polychloroprene (CR) are predominantly solvent-containing adhesives that are applied to substrates to be bonded together and are dried. By subsequently joining the substrates together under application of pressure, a joint structure is obtained with high initial strength immediately after the joining process.
For ecological reasons there is a growing need for suitable aqueous CR adhesive dispersions that can be processed to obtain corresponding aqueous CR adhesive formulations. These are used in the spray-mix process, for example, in which the aqueous adhesive formulation and a coagulant are conveyed separately in a spray gun and finally mixed in the spray jet, whereupon the CR adhesive coagulates on the substrate. An overview of this method can be found for example in “Handbook of Adhesives”, Irving Skeist, Chapman, Hall, New York, 3rd Edition, 1990, Part 15, page 301; R. Musch et al., Adhesives Age, January 2001, page 17, “Spray-Mixing Klebstoffe auf Basis Dispercoll® C für die Schaumstoff-Klebung”, Technical Information from Bayer AG, no. KA-KR-0001d/01/05.96.
It is often necessary, however, for additives such as stabilisers and/or antioxidants, for example, to be added to the aqueous CR dispersions or formulations in order to give them an appropriate storage stability and safety in use or to protect the adhesive coatings from ageing or discoloration.
For the latter purpose aqueous formulations are advantageously mixed with zinc oxide, since in formulations based on polychloroprene dispersions it counteracts rapid ageing of the glueline and discoloration of the bonded substrates due to the release of HCl from the CR polymer.
Antioxidants are also added, preferably based on oligofunctional secondary aromatic amines or oligofunctional substituted phenols such as products of the type 6-PPD (N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylene diamine; Vulkanox®, Lanxess Deutschland GmbH), DTPD, DDA, BPH, BHT, etc., as described for example in Handbuch für die Gummiindustrie, 1992 edition, Bayer AG, Leverkusen, chapter 4, Vulkanox®, p. 423. Vulkanox® DDA, a diphenylamine derivative, is particularly effective.
The zinc oxide dispersions known to date have a tendency to settle, however. This settling or phase separation is not acceptable, particularly where such adhesive formulations are used in the spray-mix process, because it causes the nozzle to clog. Cleaning the nozzle is time-consuming and cost-intensive and is unsatisfactory from an economic viewpoint.
The organic antioxidants in turn are either not resistant to discoloration (oligofunctional secondary aromatic amines) or are less effective (oligofunctional substituted phenols).
The object of the present invention was to provide novel aqueous polychloroprene dispersions and adhesive compositions which are free from residual monomer and are stable in storage and which, after application to the substrates to be bonded and joining, have a high resistance to HCl release from the polymer.